A touch screen is a device that can detect an object in contact with or in proximity to a display area. The display area can be covered with a touch-sensitive matrix that can detect a user's touch by way of a finger or stylus, for example. Touch screens are used in various applications such as mobile phones and other mobile devices. A touch screen may enable various types of user input, such as touch selection of items on the screen or alphanumeric input via a displayed virtual keypad. Touch screens can measure various parameters of the user's touch, such as the location, duration, etc.
One type of touch screen is a capacitive touch screen. A capacitive touch screen may include a matrix of conductive lines and conductive columns overlaid on the display area. The conductive lines and the conductive columns do not contact each other. The capacitive touch screen may be used for mutual capacitance sensing or for self capacitance sensing.
In mutual capacitance sensing, the capacitance between each line and column of the matrix may be sensed. A change in the sensed capacitance between a line and a column may indicate that an object, such as a finger, is touching the screen or is in proximity to the screen near the region of intersection of the line and column. Mutual capacitance sensing circuits employ a “forcing” signal applied to a column conductor of the capacitive touch matrix and sensing of the coupled signal on respective line conductors (or vice-versa).
In self capacitance sensing, the capacitance between a conductive element of the capacitive touch matrix and a reference voltage, such as ground, is sensed. A change in the sensed capacitance may indicate that an object, such as a finger, is touching the screen or is in proximity to the screen near the conductive element being sensed. The scanning of the capacitive touch matrix involves alternate sensing of the conductive lines and the conductive columns.
Mutual capacitance sensing and self capacitance sensing have advantages and disadvantages in different applications. Existing capacitance sensing circuitry is dedicated to either mutual capacitance sensing or self capacitance sensing.